One method of forming a thin sheet of glass is by a drawing process where a ribbon of glass is drawn from a reservoir of molten glass. This may be accomplished, for example, via an up-draw process, where the ribbon is drawn upward from the reservoir (e.g. Foucault or Colburn), or by a down-draw process (e.g. slot or fusion), where the ribbon is drawn downward, typically from a forming body. Once the ribbon is formed, individual sheets of glass are cut from the ribbon.
In a conventional downdraw process, the molten glass is formed into a glass ribbon contained within a draw chamber defined by a shroud that surrounds the ribbon. Among other things the shroud serves to maintain a consistent thermal environment in the region defined by the shroud and surrounding the ribbon. Roller pairs penetrate the shroud and pinch the ribbon edges. The rollers may be used to apply a pulling force to the ribbon, to apply a transverse tension to the ribbon, or merely to guide the ribbon. Accordingly, a rotational force may be applied to the rollers by a motor, or the rollers may be free-wheeling and the rotational force applied to the rollers by the descending ribbon. In either case, the rollers rotate. Production roller mechanisms typically allow for the rollers to move horizontally and/or vertically from the glass contact area. This accommodates geometric tolerances of the rolls, run-out and tolerance changes in operation, along with normal variability in glass thickness. Further, production roll mechanisms typically allow the rolls to be moved far away from the glass for maintenance access, process restart, and other practical considerations. Frictional forces that resist the free motion of the edge rollers may induce force cycling that manifests as undesirable perturbations or stress changes in the ribbon that can become frozen into the glass as the glass transitions from a viscous material to an elastic material. Another aspect of a production roller mechanism is to minimize air leakage from the draw chamber. An inadequate seal where the shaft of each roller (roll) penetrates the shroud allows excessive flows to exit the shroud—possibly over heating the surrounding equipment—and increases flows of relatively cool gas entering from the shroud bottom. High edge roll seal leakage, especially those which varies over time, can lead to non-optimal cooling of the ribbon and undesirable stress and warp in the final product. Thus, the seal must simultaneously be capable of withstanding the high temperature of the shroud and its interior environment, minimize the egress of hot atmosphere from the shroud interior, and accommodate both displacement of the roller shaft transverse to the shaft longitudinal axis and rotation motion about that axis. Currently, such seals comprise a metal-on-metal interface that introduces friction into the apparatus.